At first, Mark*, 33, thought the hard, slightly itchy spot on the ball of his left foot was just a callus. His feet weren’t used to the rigorous training he’d undertaken for his first half marathon. "The longest thing I had done prior to that was a 5K. I had no idea what to expect,” Mark tells SELF. The spot sprouted around November 2015, and with the race approaching in January, Mark wasn’t taking any chances. “I wanted to see [the half marathon] through because this was a personal goal of mine, so I erred on the side of caution and had it checked out,” he says.

That visit to the doctor’s office spiraled into a painful, exasperating saga that’s ongoing to this day. The first doctor Mark saw also chalked the spot up to the long runs Mark was doing in preparation for the race. But eventually, Mark noticed a black dot in his “callus,” and some googling revealed that he had a plantar wart. And after thinking about it, he realized the likely source was something most people have done at one point or another: going barefoot in the shower at the gym. In an effort to raise awareness, Mark anonymously sent his story to his local radio station, Houston's 94.5 The Buzz, to warn others about the dangers of forgoing shower shoes after a workout.

As anyone who’s used a gym shower knows, plenty of nastiness lurks in those stalls.

It goes far beyond soggy balls of hair, as foul as they are. When you go barefoot at gym showers or similar public facilities, you’re risking contact with bacteria, fungi, and viruses that can lead to various health conditions you’re better off avoiding.

One is athlete's foot, which causes burning, itching, redness, and skin irritation between the toes (this signals acute athlete's foot) or dryness along the soles of the feet (this points at chronic athlete's foot), Quinton Yeldell, D.P.M., cofounder of the foot-care line Southern Hospitality, tells SELF. You could also wind up with nail fungus, which typically manifests with dry, thickened, discolored nails. "Both infections are often caused by the same fungus, tinea pedis, so it is very common for someone who has athlete’s foot to also acquire nail fungus," Yeldell says.

Plantar warts, on the other hand, are caused by human papillomavirus (HPV). The virus just so happens to thrive in damp, warm environments like gym showers, meaning you can contract it if you’re sans shower shoes and there’s any point of entry on your feet, like a cut or crack.

“I’m normally very cautious about bringing shower shoes with me,” Mark says, but on a handful of occasions, he’d accidentally left them at home. “Some of the noises you hear in the men’s shower—people are blowing their noses in there and God only knows what.”

But since he worked out on his lunch break, he had a dilemma on his hands: “I had to choose to go back to work smelling really, really bad, or just suck it up and deal with showering.” He chose the latter, as his doctor later confirmed, this was likely how he wound up with a plantar wart.

Mark tried various treatments, but his situation only got worse.

Still hopeful that he could participate in his half marathon, Mark tried at-home treatments, like putting duct tape and vinegar on the wart. They brought no relief, and the wart progressed from just itching to causing a “sharp, jarring pain” whenever Mark put pressure on the ball of his foot. He realized he’d have to miss the race and progressed to more intense treatments under medical supervision, like using cantharidin, a blister-inducing medicine made with extract from beetles, and salicylic acid–based medicines.

Plantar warts tend to go away on their own, and if they don’t, treatment with a doctor's guidance often works. “The condition is usually reduced to the area of the body that was infected and does not become systemic,” Yeldell says. But even with treatment, Mark’s one wart multiplied into five. Frustrated, he switched doctors around August 2016 and tried even more aggressive treatment, using the cantharidin at a higher concentration. “It didn’t hurt right away, but within about 30 minutes, the pain would start, and in about two hours, I’d almost be in tears. It was horrible,” Mark says. He’d go in for the treatment every Friday after work because for at least a day after, he was hardly able to get his foot into a shoe, much less put pressure on it.

Eventually, the ordeal got so exhausting that Mark was ready to give up.

“I got to the point where I was like, ‘Cut off my foot, I’m done dealing with it,’” he says.

Instead, earlier this year, the doctors decided to do surgery. “Doctors initially thought the surgery would be really quick. All indications were that the plantar warts, after going through so many rounds of treatment, were fairly superficial,” Mark says. But they were deeper than the doctors realized, so excising them resulted in the gaping wounds you can see in the graphic photos below.

Mark's doctors weren't sure why his plantar warts refused to respond to treatment. Although these warts can turn into hard-to-treat infections, like methicillin-resistant Staphylococcus aureus (MRSA), his doctor knew that wasn’t it. “The only thing that ever came up was the doctor asking if I had an autoimmune disease,” Mark says. “That was the only explanation they could think of for why my body didn't respond.”

Mark is now healing and has made some promising progress.

After the surgery, he went through six weeks of an innovative skin grafting process using a $2,500 piece of dehydrated North Atlantic cod from Iceland. (He got the fish for free from Kerecis, the company behind this technology.)

“Kerecis Omega3 is intact fish skin that is rich in naturally occurring omega-3 polyunsaturated fatty acids. When grafted onto damaged human tissue such as a burn wound or diabetic wound the material recruits the body's own cells and is ultimately converted into living tissue,” the company’s website says.

This past Friday, Mark ran for the first time in a very long time. “I could only do it for a mile—it's amazing how quickly you get out of shape," he says. This weekend, he's also going to swim for the first time since this all started, and he's finally able to shower normally (he used to have a cast for his entire left leg because his foot couldn't get wet).

Although Mark is hopeful this nightmare is almost over, his doctor said they won’t know until this summer—around six months after his surgery—whether he’s in the clear.

There are a few things you can do to protect yourself from infection in public showers and locker rooms.

Yeldell recommends covering any open wounds on your feet when using a public facility, along with washing your feet daily no matter what. "Simple hygiene can work wonders to fight off the chances of infection," he says. He also suggests washing your socks after each workout: "The warmth and moisture from your feet absorbed by your socks can allow bacteria to fester."

Beyond that, he advises against sharing items like towels, razors, shoes, and socks—although MRSA is often transmitted by direct skin-to-skin contact, it can also happen via shared items.

Lastly, of course, always sport shower shoes when you're using public showers, walking around locker rooms, or lounging at public pools. "It only takes one encounter to contract an infection," Yeldell says.

It’s a lesson that, unfortunately, Mark learned the hard way. “I’ve become much more cautious,” he says. “I started going back to the gym over lunch, so I still have to shower there. But I always, always, always make sure that I have shower shoes.”

Below are some photos that portray Mark's painful experience. The images are extremely graphic.